1. Technical Field
This invention relates to a position detecting device capable of detecting plural positions indicated by electrically conductive indicators, such as fingers, using a capacitive system.
2. Description of the Related Art
In recent years, the tablet-type information terminal equipped with a touch panel has come to be widely used. In particular, advancement has been made on techniques to detect multi-touch associated with a gesture function and so forth by detecting simultaneous instruction inputs by plural fingers.
As a technique for detecting this multi-touch, the capacitive system like that disclosed in, e.g., Patent Document 1 (Japanese Patent Laid-open No. Hei 8-179871) is widely used. Specifically, the position detecting device of a touch panel disclosed in this Patent Document 1 is so configured that plural line-shape electrodes are disposed in the vertical direction and the horizontal direction of the panel surface and a predetermined drive signal is supplied to the line-shape electrodes of one of the vertical direction and the horizontal direction to obtain a reception signal thereof from the line-shape electrodes of the other. Furthermore, the intersections formed by the line-shape electrodes of the vertical direction and the horizontal direction are sequentially selected and the intensity of the reception signal is obtained to obtain the finger position based on the signal distribution thereof.
According to the position detecting device disclosed in this Patent Document 1, a signal corresponding to the finger placed only near the intersection of the selected line-shape electrode of the vertical direction and the selected line-shape electrode of the horizontal direction is detected. Therefore, even when plural fingers are simultaneously placed on the touch panel, the positions of the respective fingers can be accurately obtained without mutual interference.
The touch panel is configured by combining a position detecting device like the above-described device and a display device such as a liquid crystal display (LCD). In this case, noise generated by the display device enters the position detecting device, which frequently causes erroneous operation such as failure in correctly obtaining the position indicated by a finger and detection of an incorrect position. Therefore, in the touch panel using the capacitive position detecting device, removal of noise is an important challenge.
As the most effective method for removing this kind of noise, methods using a differential amplifier circuit are provided as disclosed in, e.g., Patent Document 2 (Japanese Patent Laid-open No. Hei 5-6153) and Patent Document 3 (Japanese Patent Laid-open No. Hei 10-20992). Specifically, two line-shape electrodes are simultaneously selected from the line-shape electrode group that obtains the reception signal among the vertical and horizontal line-shape electrodes. Furthermore, one of the selected electrodes is connected to the non-inverting input terminal (positive-side input terminal) of the differential amplifier circuit and the other is connected to the inverting input terminal (negative-side input terminal) of the differential amplifier circuit. Thereby, the noise component is canceled and only the signal difference attributed to the approach of a finger is detected.